In order for the UE to be able to send and receive data to and from the PDN, an EPS Bearer should exist between the UE and the PGW.
The PDN is the network that the UE would like to connect to.
And the EPS Bearer is a tunnel between the UE and the PGW.
The data sent between the UE, and the PDN, will move into this tunnel.
In order to well understand this topic, you need to be aware of,
LTE Network Architecture
During 'LTE attach' procedure, the network establishes this EPS Bearer between the UE and the PGW.
The PGW assigns an IP address for this Bearer. The UE uses this IP address, in order to communicate with the PDN.
In addition, the PGW assigns a QoS for this Bearer, which guarantees the QoS for the data passing in this Bearer.
During the EPS Bearer establishment, the MME assigns an 'EPS Bearer ID' that identifies this Bearers from the other Bearers.
The EPS Bearer between the UE and the PGW is composed of the e-RAB between the UE and the SGW, in addition to the S5/S8 Bearer between the SGW and the PGW.
While, the E-RAB contains the Radio Bearer between the UE and the eNodeB, in addition to the S1 Bearer, between the eNodeB and the SGW.
Default Bearer vs. Dedicated Bearer
The EPS Bearer the network establishes during LTE attach procedure, is the default Bearer.
While the network can established additional Bearers after the establishment of the default Bearer. Those Bearers are the 'Dedicated Bearers'. The Dedicated Bearers are having the same PDN connection with the same IP address as the 'Default Bearer', but the QoS assigned to the 'Dedicated Bearer', is different than the QoS assigned to the 'Default Bearer'.
Therefore, the network creates a 'Dedicated Bearer', when there is an application or service that requires special QoS characteristics.
'Default Bearer' establishment
The network establishes the default EPS Bearer during the procedure of 'LTE Attach'. The establishment occurs in below highlighted steps,
After identification and authentication of the UE. And after the MME is having the subscription information of the UE. The MME starts the establishment of the EPS Bearer as following,
- The MME sends 'Create session request' to the SGW. This includes the APN name received from the UE in the 'attach request'.
- Now, the MME validates that the UE is allowed to access this APN, by looking into the subscription information of the UE, then the MME sends 'create session request' to the SGW, including the APN that the UE should connect to .
- Then, the SGW forwards 'Create session request' to the PGW.
- The PGW assigns an IP address and QoS for this Bearer, and replies with 'Create session response'. This message includes the IP address and the QoS that the PGW assigned to this Bearer.
- Then, the SGW forwards the 'Create session response' to the MME.
- The MME sends 'initial context setup request' to the eNodeB. This message includes an 'Attach accept' message the MME sent to the the UE, including the GUTI that the MME assigned to the UE, and the IP address assigned to this Bearer.
- Now, the eNodeB establishes the radio bearer with the UE.
- The eNodeB replies the MME with 'initial context setup response'.
- And the UE replies the MME with 'Attach complete'.
- and finally the MME sends 'modify bearer request' to the SGW, and the SGW replies with 'modify bearer response'
Now, the Bearer establishment is completed, and the UE can send and receive data to and from the PDN.